<fx:root type="javafx.scene.layout.VBox" xmlns="http://javafx.com/javafx"
            xmlns:fx="http://javafx.com/fxml" minWidth="100" minHeight="20">
    <Button text="${controller.content}" minWidth="100" minHeight="20"/>
</fx:root>


public class MyButton extends VBox {

    public String getContent() {
        return content.get();
    }

    private StringProperty content = new SimpleStringProperty("Hello here");

    public MyButton() throws IOException {
            FXMLLoader loader = new FXMLLoader(getClass().getResource("MyButton.fxml"));
            loader.setController(this);
            loader.setRoot(this);
            loader.load();
    }

}